The present invention relates to lightning conductors and more specifically to lightning conductors having a piezoelectric device for starting or initiating the corona effect.
During thunderstorms, the lightning current is formed in two stages. In a first stage, a current called a "downward tracer" constituted by charged particles generally in the form of electrons is directed towards the ground. The genera direction of its movement is linked with the local electric field. At the approach of said tracer, objects on the ground are exposed to an intense electric field which, at the most favourable points, gives rise to the appearance of upward discharges, which are called "upward tracers". When an upward tracer encounters a downward tracer, this has the effect of closing an electric circuit in which the lightning current flows. The latter then falls at the point able to bring about the fastest emission of the upward tracer.
As intense electric fields preferably form in the vicinity of pointed metal objects, lightning conductors are generally constituted by a vertical rod terminated by a metal point called the "intercepting point", said rod being electrically connected to the ground. Thus, on the approach of the downward tracer, the point of the lighting conductor produces an electric field with an intensity higher than the adjacent electric field. It is therefore the preferred location for the formation of the upward tracer. The effectiveness of the lightning conductor, i.e. the extent of the protected area, increases in proportion to the emission of the upward tracer from the point being preferred compared with the emission of an upward tracer from another location.
The upward tracer results from the appearance of the corona effect consisting of an intense emission of electrons from the point or tip of the lighting conductor under the influence of an intense electric field. However, this electron emission takes place with a certain time lag (a few dozen microseconds) after the electric field has reached the value necessary for initiating the corona effect. This lag can be reduced by increasing the population of free electrons in the vicinity of the point, so that the corona effect can be formed on the basis of an electron called a "germ electron" located in the vicinity of the point. This permits the earlier formation of an upward tracer, which will meet the downward tracer at a higher altitude, so that the area protected by the lightning conductor is increased.
EP-A1-0 123 578 describes a lightning conductor in which the population of free electrons in the vicinity of the point is increased as a result of an electrode positioned in the vicinity of the point and a piezoelectric crystal which can be compressed in different ways. In order to compress the crystal, it is possible to use a flexible member moved under the action of the wind and which has a boss bearing on the crystal. It is also possible to use a turbine moved under the action of wind and which has several bosses which, during the rotation thereof, bear on one or more piezoelectric crystals, each of which is connected to an electrode. It is also possible to use an electromagnet for exerting a pressure on the piezoelectric crystal.
Although such a device makes it possible to effectively increase the population of free electrons in the vicinity of the lightning conductor point, it still suffers from certain disadvantages. Firstly, it requires high winds in the case where the compression of the crystal is obtained either by a metal plate having a boss, or by a turbine and such devices no longer operate when there is no wind or when the wind is light. Moreover, in the version using electromagnets, it is necessary to use an external power supply and the device no longer functions if the latter breaks down.